A receiver device operating at RF and microwave frequencies is not perfect. For instance the frequency response of its input channels is not ideal: the amplitude varies as function of frequency and the phase is not perfectly linear as a function of frequency. Also, the input impedance of these channels deviates from the nominal value (typically, but not necessarily 50 Ohm). In order to perform accurate measurements of unknown signals, such receiver device has to be calibrated by applying an appropriate well known signal at one or more input channels. Comparing the signal as measured by the receiver device to the known signal allows compensating for the receiver imperfections. This compensation possibly includes the effect of the input impedance and possibly cables, adapters and other means required to connect to a signal or device under test that needs to be measured.
A receiver device can be used as part of a stimulus-response system, which is used to accurately characterize a device under test. Such systems allow taking advantage of the a priori knowledge of the signals being applied. These stimulus-response systems, such as sampler-, scope- or mixer-based network analyzers, require calibration techniques in order to perform accurate measurements at one or more defined calibration planes, typically the input(s) and output(s) of the device under test (DUT). Example stimulus-response systems such as Large-Signal Network Analyzers (LSNA) or Non-linear Vector Network Analyzers (NVNA), combine an appropriate relative calibration and a power and phase calibration in order to accurately measure voltage and current (or incident and reflected waves) in amplitude and in phase at the DUT ports, at all relevant frequencies (for instance, at the fundamental frequency and at the relevant harmonics or in case of a multi-tone excitation at all fundamental tones, relevant harmonics and intermodulation products). Note that with relative calibration technique is meant that only the ratios of wave quantities (or corresponding voltage and current) at identical frequencies are calibrated.
During power calibration, a calibrated power sensor is connected to one of the calibration planes (or an auxiliary calibration plane in case of on-wafer or in-fixture measurements).
During phase calibration, a calibrated pulse generator (in the art also referred to as comb generator, phase reference or harmonic phase reference) with stable and known phase relationship between its spectral components, after eliminating an arbitrary delay, is connected to one of the calibration planes (or an auxiliary calibration plane in case of on-wafer or in-fixture measurements). This pulse generator generates at least all tones (frequency components), typically harmonics of a common low-frequency tone f0, for which the stimulus-response system needs to be calibrated. It can generate these tones simultaneously or in two or more steps, whereby each of the steps contains overlapping tones to stitch the steps together. The source driving the pulse generator must have at least a stable frequency and phase while the receivers of the network analyzer are measuring its response in a phase-coherent way. Amongst others, these pulse generators can be step-recovery-diode-based (SRD) and/or based on nonlinear transmission lines (NLTL) or based on high-speed logic.
FIG. 1 illustrates a conventional system as known in the state of the art. Two frequency-coherent sources are shown. One of the frequency-coherent sources generates a tone at a frequency n.f0, which is applied to a pulse generator (depicted as a comb generator in FIG. 1) which produces phase-coherent tones at the calibration plane of the one-port analyser. The second source, which is connected to the receiver, is used in the receiving process]. Typically the measured signal is downconverted from RF to IF (intermediate frequency) using a mixer. The second source is applied internally in the receiver to the LO (local oscillator), which is typically common to all the mixers.
As an example a fundamental frequency f0 is chosen to be 100 MHz. In order to generate phase-coherent tones at m.100 MHz (with m=1, 2, . . . , 670) one can apply a single tone at n.100 MHz to a state-of-the-art comb generator, which simultaneously and therefore phase-coherently generates all tones. This is the tone generating part. Its output is measured by a frequency-coherent receiver through signal separation hardware which allows measuring the voltage and the current or the incident and reflected wave or any combination of these quantities. The processor shown in the figure gets the measured data and determines the calibration coefficients during the calibration process. Afterwards the processing means is used to apply these calibration coefficients as part of the measurements. Typically the processor is part of the stimulus-response system, but calibration coefficients can also be extracted (and afterwards applied) using an external processing means (e.g. an external PC or laptop).
Patent application US2009/125264 presents a technique for eliminating the systematic measurement errors from a measurement system for characterizing non-linear devices. A relative and an absolute error correction are performed and the results are adapted into an error correction model. Raw measured voltage waves from the device under test are then corrected using the error correction model. The cross-frequency phase and absolute amplitude of the measured voltage waves applied to and emanating from the non-linear device are measured and error corrected with the model.
Hence, there is a need for a solution where a stimulus-response system like a network analyzer can be calibrated in a more efficient way.